Magnesium is a metal having very specific properties, and which is used in an increasing number of applications (in general in the form of alloys, particularly GA9, GA9Z.sub.1, MSR and RZ5 alloys), such properties being, for example, a favorable ratio of density to mechanical properties, ease of working, the property of damping vibrations, creep characteristics at average temperatures.
However, magnesium is the most active metal in the electrochemical series (standard electrode potential of -2.3 volts with respect to a standard hydrogen electrode at ambient temperature) and is therefor very sensitive to corrosion.
The techniques presently used for coating a magnesium based substrate with a protective layer are essentially as follows: anodization in a fluorine containing bath followed by a chromating, chemical mordanting followed by impregnation with polymers and electrodeposition of paints or varnishes. These protection techniques are effective under static conditions, that is in the absence of stressing of the surface of the substrate. By contrast, under dynamic conditions (friction, dynamic working, etc.) these protections are scratched off or are abraded, and very quickly allow the substrate to appear. Moreover, the known chromating treatments use toxic baths which lead to pollution problems which are not readily overcome at the present time.
A technique is also known for metallizing a magnesium substrate by electrolytically depositing aluminum. However, this technique leads to a poorly compacted layer of aluminum which does make the surface of the substrate less active, but which does not produce a very effective protection. Also, this layer is poorly adherent and therefor, as in the case of other coatings mentioned above, is not very effective under dynamic conditions.
Japanese patent No. A-61,190,078 proposes a process for protecting magnesium consisting of carrying out a metallization of aluminum in a vacuum on the substrate, followed by a treatment by means of a chromatizing bath. In an analogous manner, French patent No. 2,589,485 describes a magnesium treating process consisting of carrying out an ionic plating with aluminum, followed by a chromating treatment in a bath. However, in these documents, the chromating treatment is not explained in any detail, such that it is not possible to know the performance of the coatings obtained, particularly under dynamic conditions. Moreover, these protective processes, using baths of chromium salts, are subject to the same disadvantages of toxicity and pollution mentioned above.
The present invention proposes to provide a satisfactory solution to the technical problems mentioned above, for protecting a magnesium based substrate.
An essential object of the invention is to provide on the substrate a protective film which is perfectly inert in the electrochemical series so as to eliminate any risk of corrosion.
Another object is to provide a protective film of very high surface hardness, adhering very strongly to the substrate, in such a manner as to constitute an effective protection, not only under static conditions, but also under dynamic conditions (working, friction, shocks, etc.)
Another object is to permit the protection of substrates having complex surfaces by a continuous protective film.
Another object is to permit easy adjustment of the thickness of the protective film as a function of the application.
Another object is to avoid the use of toxic baths which lead to disposal and/or pollution problems.